1. Field of the Invention
The present invention relates generally to an apparatus and method for creating a preamble sequence for use in a wireless communication system, and in particular, to an apparatus and method for creating a preamble sequence indicating frame synchronization or existence of data.
2. Description of the Related Art
In general, a wireless communication system refers to a system supporting a radio communication service, and the wireless communication system includes UTRANs (UMTS (Universal Mobile Telecommunication Systems) Terrestrial Radio Access Network) and Us (User Equipments) for the wireless communication. The UTRAN and the UE perform the wireless communication using transport frames, requiring them to be synchronized with each other for exchange of the transport frames. To this end, the UTRAN transmits a sync (synchronization) signal so that the UB may recognize a start point of the frame. The UE then checks frame timing of the UTRAN by receiving the sync signal transmitted by the UTRAN.
A specific preamble sequence previously chosen by the UTRAN and the UE is typically used for the sync signal. When the preamble sequence indicating the frame timing is transmitted on a burst-by-burst basis (intermittently), not continuously, reception performance (capability) of the preamble sequence depends upon an aperiodic autocorrelation characteristic.
In addition, a sequence indicating slot synchronization is used for the preamble sequence for acquisition of the frame timing. A W-CDMA (Wideband Code Division Multiple Access) UMTS system, an FDD (Frequency Division Duplexing) UMTS system and a TDD (Time Division Duplexing) UMTS system have a plurality of slots in each frame. Therefore, the FDD UMTS system has a slot sync signal indicating a start point (synchronization) of a slot, while the TDD UMTS system has a midamble signal in every slot for synchronization of the slot. Like the frame sync signal, the slot sync signal and the midamble signal are also previously chosen by the UTRAN and the UE, and transmitted intermittently.
Further, a BRAN (Broadband Radio Access Network) system (or BRAN hyper access system) employing the W-CDMA technique transmits data to a plurality of users by dividing the frame on a time division basis. Even in the BRAN system, a frame preamble indicating a start of the frame exists in a specific period beginning at the start point of the frame. In addition, since the data for the respective users may be transmitted irregularly in one frame, a burst preamble indicating a start point of the data exists at the head of the data. Therefore, the UE should receive the data preamble in order to recognize the transmission start point of the data. That is, the UE should acquire synchronization on the start point of the data in order to receive the data. For the synchronization, the UE acquires the preamble signal used in common by every system, before receiving the data. A frame format including the preamble signal will be described with reference to FIG. 1.
FIG. 1 illustrates a common frame format used in the BRAN system. Referring to FIG. 1, a frame preamble 101 represents a start point of a frame, used in detecting synchronization of a signal from the UTRAN. The frame preamble 101 is subject to QPSK (Quadrature Phase Shift Keying) modulation before being transmitted. A broadcasting channel (BCH) 102 is used to broadcast system information required during BRAN communication to every UE in a coverage of the UTRAN. A first burst preamble 103 represents a start point of intermittently transmitted first burst data. A first data channel 104 represents a part for transmitting first data. The first data transmitted over the first data channel 104 is subject to any one of QPSK (Quadrature Phase Shift Keying), 16 QAM (16-ary Quadrature Amplitude Modulation) and 64 QAM (64-ary Quadrature Amplitude Modulation) modulations before transmission. The frame format, as illustrated in FIG. 1, includes N burst preambles and N succeeding data channels.
In the foregoing description, the system uses sequences (preambles) indicating frame synchronization, slot synchronization or existence of data, which are previously chosen by the UTRAN (transmitter) and the UE (receiver) according to the communication standard. Since the sequences are intermittently transmitted on a burst basis, the sequences have a good aperiodic autocorrelation characteristic. A structure of a common preamble transmitter will be described with reference to FIG. 2.
FIG. 2 illustrates a structure of a preamble transmitter for transmitting a preamble in a UTRAN. Referring to FIG. 2, a preamble generator 200 generates a complex preamble signal and provides the generated complex preamble signal to a first baseband filter 210 and a second baseband filter 215. Specifically, an I (In-phase) sequence signal, a real component signal of the preamble signal generated from the preamble generator 200, is provided to the first baseband filter 210, while a Q (Quadrature-phase) sequence signal, an imaginary component signal of the preamble signal, is provided to the second baseband filter 215. The first baseband filter 210 and the second baseband filter 215 filter the I signal and the Q signal provided from the preamble generator 200 into I and Q-arm baseband signals, respectively. The baseband signal output from the first baseband filter 210 is provided to a multiplier 220, while the baseband signal output from the second baseband filter 215 is provided to a multiplier 225. The multiplier 220 multiplies the signal output from the first baseband filter 210 by a carrier signal cos (2πfct), and provides its output signal to an adder 230. Further, the multiplier 225 multiplies the signal output from the second baseband filter 215 by a carrier signal sin (2πfct), and provides its output signal to the adder 230. The adder 230 adds the signal output from the multiplier 220 to the signal output form the multiplier 225, and provides its output signal to an antenna (not shown). In the conventional preamble transmitter of FIG. 2, the preamble is transmitted after being subject to QPSK modulation without any error correcting information added thereto.
Next, a structure of a common preamble receiver will be described with reference to FIG. 3.
FIG. 3 illustrates a structure of a preamble receiver in a UE, for detecting a preamble transmitted from the transmitter. In FIG. 3, an RF (Radio Frequency) part, an IF (Intermediate Frequency) part and a filtering part are omitted, for convenience sake.
Referring to FIG. 3, a received RF signal r(t) is provided to multipliers 320 and 325. The multiplier 320 multiplies the signal r(t) by a carrier signal cos2πfct for down conversion, and provides a down-converted I-component signal to a first baseband filter 310. Further, the multiplier 325 multiplies the signal r(t) by a carrier signal sin 2πfct for down conversion, and provides a down-converted Q-component signal to a second baseband filter 315. The first baseband filter 310 filters the signal output from the multiplier 320 and provides its output signal to a matching filter 300 as an I-component signal. The second baseband filter 315 filters the signal output from the multiplier 325 and provides its output signal to the matching filter 300 as a Q-component signal. A preamble generator 330 creates an I-component preamble signal and a Q-component preamble signal and provides the created preamble signals to the matching filter 300. The matching filter 300 detects a correlation between the I and Q-component signals output from the first and second baseband filters 310 and 315 and the I and Q-component preamble signals output from the preamble generator 330, and then provides the detected correlation value to a decision part 340. The decision part 340 compares the correlation value output from the matching filter 300 with a unique absolute threshold previously set in the receiver. As the result of the comparison, if the correlation value output from the matching filter 300 is higher than or equal to the threshold, the decision part 340 outputs a preamble acquisition indication signal. Otherwise, if the correlation value output from the matching filter 300 is lower than the threshold, the decision part 340 outputs a preamble acquisition failure signal.
The conventional receiver, as described above, uses the correlation characteristic in order to detect the preamble. In this case, the preamble detection performance depends upon the aperiodic autocorrelation characteristic of the preamble. Therefore, as mentioned above, it is necessary to use a code having a good aperiodic autocorrelation characteristic for the preamble signal aimed at reception synchronization.
As described above, the preamble used in the BRAN system is classified into a frame preamble for indicating a start point of one frame and a burst preamble for indicating a transmission start point of burst data. A downlink frame preamble signal among the frame preambles should have a length of at least 32 bits, and a downlink burst preamble signal among the burst preambles should have a length of at least 16 bits. In addition, an uplink burst preamble signal among the burst preambles should have a length of at least 32 bits. That is, even the same system requires the preamble signals having various lengths. In creating the preamble signals having various lengths, it is preferable to use a common preamble generator rather than using a plurality of separate preamble generators for creating preamble signals having different lengths.